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Roy, A., Royer, A., Derksen, C., Toose, P., Brucker, L., Mialon, A., Lanlois, A., and Kerr, Y.H. (11-Apr-16)

Snow and frozen ground play a crucial role in climatological and hydrological processes, and are key factors in modulating energy, water, and carbon budgets. L-band space-borne missions such as Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), and Aquarius have the potential to provide enhanced information on the surface freeze/thaw (F/T) state over northern regions. It is a key parameter for studies of terrestrial hydroclimatology and ecosystem processes. In this presentation, we first present an analysis over Canadian sites of the L-band brightness temperature (TB) variations from SMOS, Aquarius and first SMAP data in order to characterize the freeze/thaw (FT) soil state, including in winter when a dry snow cover exists. The analysis shows that because of the strong permittivity difference between ice and water, the signal at L-band is very sensitive to the F/T state. However for forested sites it was shown that the signal change is more subtle during transitional periods, suggesting that the boreal F/T signal is more ambiguously influenced by the vertical soil-vegetation continuum and its developmental stages following vegetation phenology. Despite the strong sensitivity of the L-Band passive microwave signal to soil F/T, the intensity can vary because of spatially varying contributions from vegetation, soil, lake and snow, and their physical properties. Hence, a second analysis was conducted at local scale from surface-based L-band radiometer measurements in Saskatchewan, Canada during the 2014-2015 fall, winter, and spring. This dataset was used to analyse the effects of frozen soil and snow. The results show that the snow has a non-negligible effect on TB at L-Band. This study brings important information for the development and improvement of F/T algorithm from SMAP and SMOS observations. The work confirms that spaceborn L-Band data can also leads to retrieve other important cryospheric variables such as snow density (Lemmetyinen et al., 2015).